This invention relates to variable resistors of the kind (herein referred to as the kind specified) comprising an arcuate resistor element applied on a substrate forming part of a housing for a rotor carrying a wiper contact which contacts the resistor element and, as the rotor rotates, varies a tapping point on the resistor element, the rotor being formed peripherally with gear teeth meshing with a worm by which the rotor is rotated. Such resistors may be used as trimmer potentiometers and are usually of small dimensions.
Desirably a variable resistor of the kind specified is provided with abutments limiting the extent of rotation of the rotor to less than 360.degree. and also with a "clutching" arrangement which allows slipping between the rotor gear teeth and the worm if the latter is rotated in a manner tending to rotate beyond the limits of the prescribed extent of rotation, thereby to avoid damage to the resistor by forcible operation of the worm gear. In one form of such clutching arrangement (see U.S. Pat. Nos. 3,416,119 and 3,446,085) the rotor is of an elastomeric material and has a minor part of its gear-toothed periphery separated from the central boss of the rotor by circumferentially-extending through slotting of the rotor so that the minor part is more radially deflectable than the remainder of the periphery; the minor part may have one end divided off from the remainder of the periphery or may be divided into two at its middle.
Such clutching arrangements, while allowing satisfactory clutching, have disadvantages in manufacture and use. Thus in moulding the rotor one of the faces of the minor peripheral part must be a "shut-off" face in the mould so that a flash may be produced which would adversely affect its operation. Also in order to allow correct clutching, the minor part has an extent comprising several gear teeth with the result that poor engagement between the gear teeth and the worm is possible away from the limiting positions of travel of the rotor where the clutching action is not required, leading to inaccurate or poor setting of the device.